The present invention relates to the trimming of a device for use in microcontroller-based products. More particularly, the present invention relates to a method and circuit for effectively trimming offset and temperature drift in operational amplifiers and voltage references.
The demand for higher performance, microcontroller-based products for use in communication and processing applications continues to increase rapidly. As a result, microcontroller and electronics based product manufacturers are requiring for the components and devices within these products to be continually improved to meet the design requirements of a myriad of emerging audio, video, imaging and wireless applications. These microcontroller-based products comprise devices such as, for example, operational amplifiers (xe2x80x9cop ampsxe2x80x9d) and voltage references. Op amps provide, for example, signal amplification and high impedance signal transfer. Voltage references provide substantially constant output voltages despite gradual or momentary changes in input voltage, output current or temperature.
Op amps, voltage references and other similar types of devices typically are trimmed during or after manufacture to improve the precision and accuracy of the devices. For example, the trimming of differential input stages of an op amp can be performed such that equal input signals applied to both input terminals of the op amp generates equal output signals at both outputs of the op amp. Microcontroller-based products incorporating circuits which benefit from trimming techniques include various digital devices, such as clock radios, microwave ovens, digital video recorders and the like.
Some of the main objectives for trimming techniques include the correction of offset and temperature drift in the op amp. Offset and temperature drift can arise due to manufacturing variation, inherent temperature mismatches at the manufacturing level, and packaging stresses. Utilizing trimming techniques in correcting for these offsets and temperature drifts improves the accuracy and reliability of the circuit. In bipolar op amps, the input stage offset is directly correlated with temperature drift. Therefore, offset trimming performed at normal temperatures can improve the temperature drift parameter as well. However, bipolar reference devices do not have such features and therefore, simultaneous output voltage and drift trim is not possible using this same technique on bipolar reference devices.
In field effect transistor (xe2x80x9cFETxe2x80x9d) op amps, the input stage offset can be correlated to the temperature drift by measuring the offset at two different temperatures and calculating the amount of trimming needed. This trimming calculation method, however, is complex because the trimming procedure is unique to the individual part, and thus part identification is needed during both measuring and trimming of the offset. For this reason, this trimming method is not suitable for in-package trimming during final testing of the op amp. In CMOS op amps, no correlation exists between offset and temperature drift. Furthermore, relatively large offset/temperature drift shifts can result from packaging a CMOS circuit. Therefore, it is impractical to perform trimming at the wafer-level, i.e., performed before packaging.
Accordingly, a need exists for an improved method and circuit for trimming circuits to effectively address both offset and temperature drift problems.
The method and trimming circuit according to the present invention address many of the shortcomings of the prior art. In accordance with one aspect of the present invention, a trimming circuit is provided for offset trimming for at least two different temperatures, wherein the offset trimming can effectively decrease both offset and temperature drift in op amp and voltage reference devices. In accordance with another aspect of the present invention, changing the temperature dependencies of current sources facilitates the trimming processes. In accordance with an exemplary embodiment of the present invention, the trimming circuit is configured with a current source stage configured to trim the output of an op amp or voltage reference stage by changing the temperature dependencies of various current sources within the current source stage.
In accordance with an exemplary embodiment of the present invention, the trimming circuit comprises at least two current sources configured such that one current source has a positive temperature coefficient and another current source has a negative temperature coefficient. In accordance with this embodiment, the two current sources are configured to be interchangeable with other current sources to facilitate changing of the temperature coefficient of one of the two current sources. In accordance with this embodiment, the two current sources are also configured to be selectively connected to the op amp input stage offset-control terminal or terminals or voltage reference input stage voltage-control terminal or terminals.
In accordance with another exemplary embodiment of the present invention, the trimming circuit is configured with one or more combination current sources comprising at least two parallel current sources. In accordance with this exemplary embodiment of the present invention, the one or more combination current sources are configured to selectively connect to, or disconnect from, either of the offset-control terminals of the op amp device, e.g., a differential input stage, to provide trimming. In accordance with an exemplary embodiment, at least one of the two current sources within a combination current source is configured to be interchangeable with other current sources to facilitate the changing of the temperature coefficient of that current source and to thus facilitate offset and drift trimming.
In accordance with another aspect of the present invention, a method of trimming is provided for offset trimming a an op amp or voltage reference device on at least two different temperatures wherein the trimming method effectively decreases offset as well as temperature drift. In accordance with an exemplary embodiment of the present invention, the trimming method comprises an exemplary first step, executed at a first temperature, of trimming first and second current sources, having first and second temperature dependencies, to balance the currents of the first and second current sources. During a second exemplary step, and at the first temperature, the output of an op amp device, e.g., a differential input stage, is trimmed. In accordance with an exemplary embodiment of the present invention, the step of offset trimming can be performed at the first temperature by switching combination current sources from one output of the differential input stage to another output of the differential input stage. In another exemplary embodiment of the present invention, the offset trimming can be performed by adding or removing combination current sources from an output of the differential input stage. Upon trimming the differential input stage, the trimming method further employs the step of offset trimming at a second temperature. In accordance with an exemplary embodiment, the step of offset trimming at a second temperature can be performed by changing the temperature dependency of at least one current source within one or more combination current sources. In accordance with an exemplary embodiment of the present invention, the temperature dependency change is facilitated by replacing a first current source with a different current source having the same temperature coefficient as the second current source.
In a further exemplary embodiment, individual first and second current sources, having first and second temperature coefficients respectively, are configured such that at least some of the first and second current sources may be individually connected to, or disconnected from, an output of the differential input stage, while some of the first and second current sources may be switched from one output of the differential input stage to another. In an exemplary first step, and at a first temperature, the current sources are trimmed for equal current between first and second current sources. In an exemplary second step, and at the first temperature, first current sources are added and removed from the outputs of the differential input stage to provide offset trimming. In an exemplary third step, and at a second temperature, some first current sources having a first temperature coefficient are replaced with other current sources a temperature coefficient similar to the second current sources for facilitating offset trimming the output terminals.